1. Field of the Invention
This invention relates to re-adjustable planar configured resistive elements and, more particularly, to methods of manufacturing and resistance trimming those of the metallized film type.
2. Background of the Invention
In many integrated and hybrid integrated circuits, including those of the thin and thick film types, there has been a need for planar configured metallized resistors. Regardless whether such resistors are of the integrated circuit diffused type, or of the plated, etched foil or thin or thick film type, there has often been a need to adjust such resistors (all generally referred to herein as being of the metallized type) after their complete fabrication as an integral part of a composite circuit. In a typical application, the "as fabricated" value of resistance of a given metallized resistor may often have to be adjusted either upwardly or downwardly in value to optimize the operating performance, or to balance, or to otherwise satisfy a critical operating requirement, of a given associated device, or circuit, or sub-circuit within a larger composite circuit.
The need for resistance trimming metallized resistors of relatively high value, in particular, is due in large measure to the fact that whether they are made as a diffused element in an epitaxial layer of an integrated circuit, or as thin or thick film elements fabricated on the surface of a ceramic substrate, variations from a desired nominal value may often be as large as plus or minus 25%. Such deviations are generally experienced because of the many processing variables that cannot, at least at present, be controlled within stringent limits, particularly in high volume production operations.
With respect to adjusting the resistance of filmtype resistors in particular, they may be readily increased by utilizing conventional techniques, such as selective anodization, chemical etching or laser or mechanical machining. Alternatively, the nominal resistance of such resistors may also be increased by forming the resistive element with one or more parallel current-steering paths and, thereafter, selectively forming open circuits along selected ones of such paths, as disclosed in U.S. Pat. No. 4,041,440, of J. L. Davis.
A similar technique is disclosed in U.S. Pat. No. 3,930,304, of H. Keller et al., wherein a plurality of serially connected diffused resistors forming part of an integrated circuit are each short-circuited by a different metallized shunt, as fabricated. Each shunt is adapted to be selectively opened by a pulse of current applied therealong, with the particular shunts opened determining the final value of total series resistance. In order to adjust the total series resistance both upward and downward utilizing this technique, each resistor must be manufactured with a different nominal value of resistance. Unfortunately, this may not always be desired, or possible, in many circuit applications.
A technique for effecting either upward or downward trimming of individual film-type resistors is disclosed in U.S. Pat. No. 3,071,749, of J. E. Starr. in accordance with that disclosure, an insulative layer is interposed between a foil-formed patterned resistor and a planar metal sheet, with one terminal connection being made to the resistor at one end and either at a point on the metal sheet adjacent the opposite end of the resistor, or to both the resistor and the sheet at the opposite end of the resistor. In the latter case, only a downward change in resistance may be produced by establishing an electrical connection between a given point intermediate the ends of the resistor and an underlying point on the metal sheet. This is accomplished by removing a discrete area of the insulative layer disposed between the patterned resistor and metal sheet, and then making a permanent solder connection therebetween. Should the adjusted value of resistance be too low, then the initially established solder connection must be eliminated, and a new one established at a slightly different location. Unfortunately, any previously established soldered connection of the type in question is not readily eliminated, because the discrete area of the intermediate insulative layer necessarily removed for that purpose now exposes the resistor to the metal sheet. The resistance trimming technique in question is thus less than completely desirable from a reversability standpoint. In addition, locating the precise point on the insulated underlying metal sheet where a short-circuit connection should be made with the overlying resistor, in order to produce a value of resistance within stringent limits, is not always readily determinable without a certain amount of trial and error.
In view of the foregoing, in many circuit applications wherein the "as fabricated" nominal value of resistance exhibited by a metallized resistor must be lowered, in particular, to balance the internal resistance or impedance of an associated discrete circuit device, for example, it has often been necessary, or at least most expedient from an alternative standpoint, to simply match such circuit devices, through a sorting process, with the respectively associated resistors, as fabricated. In large volume manufacture, however, the need to stock and sort large volumes of circuit devices according to their internal resistance (or impedance) values is both costly and time consuming.